The present invention relates to a semiconductor laser device, more specifically a semiconductor laser device capable of realizing high power and high reliability, and to an optical disk recording and reproducing apparatus using the same.
In recent years, with demands for faster and larger-capacity semiconductor laser devices applied to optical communications devices and optical recording apparatuses, research and development have been promoted for improving various properties of the semiconductor laser devices.
Among the semiconductor laser devices, those having an oscillation wavelength of 780 nm band for use in optical disk reproducing apparatuses and optical disk recording and reproducing apparatuses such as CD and CD-R/RW are conventionally made of AlGaAs based materials and typically have ridge stripe shape.
Generally in such semiconductor laser device, in superimposing a current constriction layer, a portion in the vicinity of a lateral face of a ridge stripe is positioned below an overhang of a contact layer, which prevents material gas from sufficiently reaching the vicinity of the lateral face of the ridge stripe. Further, due to plane orientation of the lateral face of the ridge stripe, there is an area whose crystal growth rate is slow. As a result, the portion in the vicinity of the lateral face of the ridge stripe is not fully filled up and a hollow portion is generated therein.
The above has been disclosed in Japanese Patent Laid-Open Publication HEI No. 3-64980, in which a means for eliminating the hollow portion has been proposed to solve a problem that the hollow portion has a low refractive index and therefore a single transverse mode oscillation is difficult to produce, and the like. A schematic view thereof is shown in FIG. 8, with reference to which outlined description will be made hereinbelow.
The semiconductor laser device is so structured that on top of a GaAs substrate 501, there are laminated in sequence an AlGaAs first cladding layer 502, an AlGaAs active layer 503, an AlGaAs second cladding layer 504, and a GaAs contact layer 505. Further, there is spattered an SiO2 film (unshown), which is formed into a stripe shape by a usual photo step. Then, with the SiO2 film as a mask, the contact layer 505 and the second cladding layer 504 are etched by chemical etching to make the second cladding layer 504 into ridge stripe shape.
With the SiO2 film as a mask for selective growth, there is formed a GaAs current constriction layer 506 on the both sides of the ridge stripe-shaped second cladding layer 504. After that, the SiO2 film is removed and the other contact layer 505 is laminated on the entire surface of the already formed contact layer 505 and the GaAs current constriction layer 506 so that the laminated contact layer 505 is integrated with them.
In the above conventional example, the hollow portion is eliminated to stabilize transverse mode oscillation. However, an inventor of the present invention actually manufactured as an experiment an AlGaAs based high-output semiconductor laser device based on the conventional technique, as a result of which it was confirmed that a maximum optical output thereof is approx. 180 mW, and end face destruction occurs at this optical output level. This is because the presence of active Al tends to generate Al oxide on a laser end face, which prevents implementation of higher output, higher reliability and longer life.
Also in the above conventional example, the contact layer 505 and the second cladding layer 504 are etched into ridge stripe shape with an etchant modified to prevent the stripe-shaped contact layer 505 from protruding from the ridge-strip-shaped second cladding layer 504 in lateral direction like an overhang. This method, however, suffers difficulty in management of etchant and etching time.
Accordingly, it is an object of the present invention to provide a high-output semiconductor laser device using a GaAs substrate, more specifically a 780 nm-band high-output semiconductor laser device for use in CD-R/RW and the like capable of implementing a single transverse mode oscillation and also implementing high reliability and long life in high-output driving state, as well as to provide an optical disk recording and reproducing apparatus with use of the semiconductor laser device.
In order to accomplish the above object, there is provided a semiconductor laser device comprising in sequence on a GaAs substrate: a first cladding layer having a first conduction type; a quantum well active layer; a second cladding layer having a second conduction type; and a ridge stripe-shaped third cladding layer having a same conduction type as the second cladding layer, as well as a current blocking layer having a first conduction type located on both sides of the third cladding layer,
the quantum well active layer being structured from III-V group compound semiconductor containing at least P as V group element,
the first cladding layer, the second cladding layer, the third cladding layer, and the current blocking layer being structured from III-V group compound semiconductor containing only As as V group element, and wherein
a hollow portion is provided inside the current blocking layer in the vicinity of and approximately parallel to the ridge stripe-shaped third cladding layer.
According to the above configuration, there is implemented a 780 nm-band high-output semiconductor laser device having stabilized transverse mode oscillation, high reliability in high output operation and long life. This is because in the quantum well active layer having an oscillation wavelength of 780 nm band, III-V group compound semiconductor containing P, e.g. InGaAsP based compound semiconductor, has a refractive index smaller than that of AlGaAs based compound semiconductor. More particularly, use of, for example, InGaAsP based materials in the quantum well active layer decreases difference in refractive index between the hollow portion and the quantum well active layer compared to the case of using an active layer made of conventional AlGaAs based materials, which generates acceptable difference of refractive index sufficient for stabilizing a single transverse mode oscillation.
Also in the semiconductor laser device, the hollow portion formed inside the current blocking layer saves an effort at preventing an overhang formed over the ridge stripe-shaped third cladding layer, which facilitates management of etchant and etching time for forming the ridge stripe-shaped third cladding layer.
In one embodiment, right above the ridge stripe-shaped third cladding layer, there is laminated a cap/intermediate layer having a width larger than a width of a lowermost portion of the ridge stripe-shaped third cladding layer.
According to the above embodiment, the hollow portion is located in more suitable position for stabilizing transverse mode oscillation in high-output driving state.
In one embodiment, the ridge stripe-shaped third cladding layer has a reverse mesa shape in cross section.
The reverse mesa shape in cross section herein refers to the shape of a cross section vertical to extending direction of the ridge stripe-shaped third cladding layer, in which the width of the ridge stripe-shaped third cladding layer is narrowed toward the GaAs substrate, or narrowed in the middle.
According to the above embodiment, the ridge stripe-shaped third cladding layer has a reverse mesa shape in cross section, so that the hollow portion is formed in an optimum position. This may provide a semiconductor laser device implementing stabilized transverse mode oscillation in high-output driving state as well as having high reliability and long life.
In one embodiment, a width of the semiconductor layer right above the ridge stripe-shaped third cladding layer is larger than a width of a lowermost portion of the ridge stripe-shaped third cladding layer in a range from 0.48 xcexcm to 1.08 xcexcm in one side.
According to the above embodiment, a width of the semiconductor layer right above the ridge stripe-shaped third cladding layer is larger than a width of a lowermost portion of the ridge stripe-shaped third cladding layer in a range from 0.48 xcexcm to 1.08 xcexcm in one side, which makes it possible to form the hollow portion of an optimum size in an optimum position. Therefore, there may be provided a semiconductor laser device implementing stabilized transverse mode oscillation in high-output driving state as well as having high reliability and long life.
In one embodiment, the third cladding layer and the current blocking layer are provided on an etching stopper layer, and distance between the hollow portion and the etching stopper layer is 0.3 to 0.6 xcexcm.
Herein, the distance between the hollow portion and the etching stopper layer refers to the distance between the lower edge of the hollow portion and the upper face of the etching stopper layer.
In this embodiment, the distance between the hollow portion and the etching stopper layer is set to 0.3 xcexcm or above, which makes it possible to prevent light confining effect from becoming too strong and to restrain absorption of laser light into the GaAs substrate. The distance is also 0.6 xcexcm or less, which prevents optical effects such as light confining effect from becoming too weak. According to the embodiment, therefore, there may be provided a high-output semiconductor laser device having high reliability.
In one embodiment, the current blocking layer is present between a lateral face of the ridge stripe-shaped third cladding layer and the hollow portion, so that the lateral face of the ridge stripe-shaped third cladding layer is not exposed to the hollow portion.
According to the above embodiment, the lateral face of the ridge stripe-shaped third cladding layer is not exposed to the hollow portion, which makes it possible to prevent the lateral face of the third cladding layer from oxidizing, thereby contributing to increase of reliability and life of a semiconductor laser device.
In one embodiment, a lowermost portion of the ridge stripe-shaped third cladding layer has a width of 1.5 to 3.0 xcexcm.
According to the embodiment, a lowermost portion of the ridge stripe-shaped third cladding layer has a width of 1.5 to 3.0 xcexcm, which enables more stable single transverse mode oscillation of laser light.
In one embodiment, the quantum well active layer includes at least a well layer and a barrier layer, and at least the well layer is composed of InGaAsP.
According to the above embodiment, there is provided a 780 nm-band high-output semiconductor laser device in which the transverse mode oscillation is stabilized and which has high reliability and long life in high-output driving state.
In one embodiment, the quantum well active layer is a pseudomorphic quantum well active layer.
According to the above embodiment, the quantum well active layer is the pseudomorphic quantum well active layer, which further makes it possible to obtain a semiconductor laser device with lower threshold current value and higher output.
In one embodiment, the well layer in the quantum well active layer has compressive strain.
According to the above embodiment, the well layer in the quantum well active layer has compressive strain, so that a 780 nm-band semiconductor laser device may be realized with use of, for example, a compressive strained quantum well active layer made of InGaAsP on the GaAs substrate. Since the compressive pseudomorphic quantum well active layer made of InGaAsP is a quantum well active layer without the presence of Al unlike AlGaAs, high output is achievable. Further, the presence of the hollow portion makes it possible to provide a semiconductor laser device with higher reliability and higher output.
In one embodiment, a percentage of compressive strain present in the well layer is within 3.5%.
According to the above embodiment, a percentage of compressive strain present in the well layer is within 3.5%, which makes it possible to implement a semiconductor laser device with higher output, higher reliability and longer life.
In one embodiment, the barrier layer in the quantum well active layer has tensile strain.
According to the above embodiment, the barrier layer in the quantum well active layer has tensile strain, which makes it possible to compensate compressive strain present in the well layer. This enables formation of a pseudomorphic quantum well active layer having more stable crystal, resulting in implementation of a semiconductor laser device with higher reliability.
In one embodiment, a percentage of tensile strain present in the barrier layer is within 3.5%.
According to the above embodiment, a percentage of tensile strain present in the barrier layer is within 3.5%, which implements a semiconductor laser device with higher output, higher reliability and longer life.
In one embodiment, the first cladding layer is composed of two AlGaAs layers each having different Al mixed crystal ratios, out of which one layer closer to the quantum well active layer has higher Al crystal mixed ratio than that of the other layer.
According to the above embodiment, out of two AlGaAs layers having different Al mixed crystal ratios that constitute the first cladding layer, the layer closer to the quantum well active layer has higher Al mixed crystal ratio than that of the other layer. This enables effective sealing of a laser light escaping toward the GaAs substrate, thereby further restraining adsorption of light into the GaAs substrate in high output operation. Therefore, still higher output and higher reliability of the semiconductor laser device may be achieved.
An optical disk recording and reproducing apparatus of the present invention has any one of the above-described semiconductor laser devices.
The semiconductor laser device is used in the optical disk recording and reproducing apparatus, which enables high speed reading and writing operation.